A 3D printer is a cool tool. Some might say it’s a cool toy. Some might say it’s just a gadget. What most owners will say is that it is a machine that builds machines. Better machines with every iteration. ‘Iteration’ is a cool word though.
What most people don’t get is the capability of printing complex, fine mechanical or even impossible to make intricated structural elements. Things only skilled precision mechanics would be able to produce.
Takes this part of the Rostock Mini for example.
It has integrated hex nut slots.
Slots for linear bearings.
Flush-mounts for cylinder bolts.
Try this to build out of wood or plastic and you will soon hit the wall in your somewhat limited garage.
This is exactly the point where 3D printers jump in and save you a ton of work, giving you the ability to build stuff that allows you to go where no diy garage project has gone before.
But the question “What do you want to print with it” still arises as you admire the machine on your desk.
Builing a new 3D printer is why those (end-customer) machines were made.
2nd raison d’être is to rapid prototype (industry) though the big companies don’t fiddle around with FDM (Fused Deposition Modeling). They go for STL (Stereolithographie).
And then of course there is the need to build stuff that only existed in your imagination like a smart-watch. But not one of those tiny, shiny, cute electronic scrap watches. I am speaking of a BAD-ASS un-sublte smartwatch like Zack built one. Of course it has a laser. So all your question about the why are hereby swept-away. Check out his blog.